While we can read about the Sabbath in the New Testament, it does not take on the same significance as the first day of the week. The Sabbatarian will usually argue that the Roman Catholic Church changed the day of worship from the seventh day to the first day. For example, Seventh Day Adventism has this to offer:

"In the early part of the fourth century, the emperor Constantine issued a decree making Sunday a public festival throughout the Roman Empire." (The Great Controversy, Ellen G. White, p53)

"It was in behalf of the Sunday that popery first asserted its arrogant claims; and its first resort to the power of the state was to compel the observance of Sunday as 'the Lord's day' " (ibid., p446f)

"When the Sabbath was changed by the papal power, the seal was taken from the law." (ibid., p452)

When we look back at history, it becomes clear that the Sabbatarian is grasping at straws with this argument. Note that the following quotes from early authors are all made before the alleged actions of any pope:

250-325 AD - "On the day of the resurrection of the Lord, that is, the Lord's day, assemble yourselves together, without fail." (Constitutions of the Holy Apostles, Ante-Nicean Fathers, V 7, p471

145-220 AD - "If any indulgence is to be granted to the flesh, you have it. I will not say your own days, but more too; for tot the heathens each festive day occurs but once annually: you have a festive day every eighth day." (Tertullian, Ante-Nicean Fathers, V 3, p70)

ca. 200 AD - "Wherever we are, we are all called after the one name of Christ -- Christians. On one day, the first of the week, we assemble ourselves together..." (Bardesanes, Ante-Nicean Fathers, V 8, p733)

130-135 AD - "Wherefore, we (Christians) keep the eighth day for you, on which also Jesus arose from the dead and when he appeared ascended into heaven." (Barnabas, 15:9)

110-165 AD - "But Sunday is the day on which we all hold our common assembly, because it is the first day on which God, ... rose from the dead. For He was crucified on the day before that of Saturn; and on the day after that of Saturn, which is the day of the Sun, having appeared to His apostles and disciples, He taught them these things, which we have submitted to you also for your consideration." (Justin Martyr, Ante-Nicean Fathers, V 1, p186)

110-165 AD - "The commandment of circumcision, requiring them always to circumcise the children on the eighth day, was a type of the true circumcision by which we are circumcised from error and evil through the resurrection from the dead on the first day of the week of Jesus Christ our Lord. For the first day of the week, although it is the first of all days, yet according to the number of the days in a cycle is called the eighth (while still remaining the first." ( Justin Martyr, Dialogue, 41:4)

30-107 AD - "And after the observance of the Sabbath, let every friend of Christ keep the Lord's Day as a festival, the resurrection-day, the queen and chief of all the days (of the week)." (Ignatius, Ante-Nicean Fathers, V 1, p63)

When did this idea of Sunday worship arise? If we look back to scripture, we find a very clear answer:

First we find that the church began on Pentecost. Pentecost fell on the first day of the week (which Justin Martyr alluded to in the quote above). So, the church finds it's beginning on Sunday (see Acts 2:1, 41, 47; also Lev 23:15f). Surely this makes some sense to us when we remember that Jesus was raised from the dead on the first day of the week (Mt 28:1ff).

Take note of the fact that the Lord's supper was observed on the first day of the week:

"Now on the first day of the week, when the disciples came together to break bread, Paul, ready to depart the next day, spoke to them and continued his message until midnight." (Acts 20:7)

Compare this to Paul's instructions to the Corinthians:

"Therefore when you come together in one place, it is not to eat the Lord's Supper. For in eating, each one takes his own supper ahead of others; and one is hungry and another is drunk. What! Do you not have houses to eat and drink in? Or do you despise the church of God and shame those who have nothing? What shall I say to you? Shall I praise you in this? I do not praise you. For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread; and when He had given thanks, He broke it and said, 'Take, eat; this is My body which is broken for you; do this in remembrance of Me.' " (I Cor 11:20-24)

Christians partook of the Lord's supper when they assembled together. When did they do this? We read of no day other than Sunday for that feast. Inasmuch as Luke says they waited (v6), and then came together on "the first day of the week", a weekly observance is clearly implied. A monthly observance would have been "the first Lord's Day of the month". Just as the Sabbath was observed every Saturday under the old law (even though God didn't have to say "every"); so also the Lord's Day is observed as often as it arrives in the calendar. And the worship that attends that day should also be observed just as often.

It is also important to examine the instructions given in I Corinthians 16:1 & 2:

"Now concerning the collection for the saints, as I have given orders to the churches of Galatia, so you must do also: On the first day of the week let each one of you lay something aside, storing up as he may prosper, that there be no collections when I come."

Why do so on the first day of the week? Something was to be laid aside on that day, and some have foolishly supposed that a laying aside at home would have sufficed. But that would defeat the purpose for which Paul gave this instruction! He said to do it "that there be no collections when I come". If these people are laying it aside at home, then there still must be a collection when he comes. Let's allow Paul to make sense here! It is evident that the brethren were gathering on the first day of the week, as we have already noted. Such would be the natural time to gather collections into a common fund. Then when Paul comes, it is ready for him to deliver to the needy saints.

Conclusion...

The Sabbath was not the Lord's day in the first century. The Lord's day was the day on which Jesus was raised from the dead. And that was the day after the Sabbath -- the first day of the week. We worship on that day, not because the Pope decreed it, but because God decreed it.